1. Field of the Invention
The present invention relates to a method for manufacturing fine structures such as diffraction grating and the like.
2. Description of Related Art
Known binary optics manufacturing methods wherein a resist is formed into a step-like formation by means of electron beam exposure and developing, and using the structure as a diffraction pattern (diffraction grating), is disclosed in the Electronic Communications Journal (c) J66-CP85-91 January 1983, Japanese Patent Laid-Open No. 62-256601, Japanese Patent Laid-Open No. 62-42102, and so forth.
Also, Japanese Patent Laid-Open No. 61-137101 discloses art wherein two or more types of films with etching endurance are layered to a certain thickness, and sequentially etched from the upper layer so as to form a stair-like formation, which is made to serve as a mold for diffraction optical devices. Also, Japanese Patent Laid-Open No. 61-44628 and Japanese Patent Laid-Open No. 6-160610 disclose art wherein resist is formed on a substrate one step at a time as an etching mask to form a stair-like formation, which is made to serve as a mold for diffraction optical devices.
Further, Japanese Patent Laid-Open No. 8-15510 discloses art wherein one each of a etching stopper layer and a transparent layer are layered on a substrate, so as to directly form a stair-like formation, by means of alignment, exposure, and etching, which is made to serve as a diffraction optical device.
Also, Japanese Patent Application No. 6-26339 and U.S. Pat. No. 5,324,600 disclose art wherein alignment is performed each time the resist is patterned, thus forming a stair-like structure of resist directly in the substrate as an etching mask, which is made to serve as a diffraction optical device. Japanese Patent Laid-Open No. 7-72319, which corresponds to Application No. 6-26339, discloses art for forming a stair-like structure by performing alignment with the resist as an etching mask.
FIG. 26 is a cross-sectional diagram illustrating the manufacturing process of a diffraction optical device of an 8-stepped structure. In Step (1) of FIG. 26, resist is dropped onto a cleansed substrate 1, and the resist is formed into a thin film of 1 μm in thickness by means of spin-coating, which is then baked to form the resist film 2. In Step (2) of FIG. 26, the substrate 1 is mounted to an exposure device capable of exposing the finest diffraction grating pattern into the resist film 2, and exposure light L with sensitivity to the resist film 2 is cast thereupon with a reticle 3 formed with a pattern according to the desired diffraction grating pattern serving as a mask, thereby performing exposure. In the event that a positive-type resist is used, the area exposed by the exposure light L becomes soluble to the developing agent, and so a resist pattern 4 with the certain dimensions is formed, as shown in Step (3) of FIG. 26. In Step (4) of FIG. 26, the substrate 1 is mounted to a responsive ion etching device or an ion beam etching device capable of anisotropic etching, and the etching is performed to the substrate 1 for a certain time to a certain depth, with the patterned resist 4 as the etching mask. Then, removing the resist pattern (4) yields the substrate 1 formed with a pattern 5 having a two-stepped stair-like pattern, as shown in Step (5) of FIG. 26.
Again, in Step (6) of FIG. 26, a resist film 6 is formed on the substrate 1 as with Step (1) and mounted on the exposing device, and following alignment with alignment precision that the exposing device has to the pattern so far formed, with the reticle 7 having a two-fold cycle pattern of the diffraction pattern, following which the resist film 6 is exposed and developed to form an alignment pattern 8 in Step (7) of FIG. 26. Next, removing the resist pattern following dry etching as with Step (4) yields a four-stepped stair-like pattern 9, as shown in Step (8) of FIG. 26.
Further, in Step (9) of FIG. 26, a resist film 10 is formed on the substrate 1 as with Step (1), and with the reticle 11 having a four-fold cycle pattern of the diffraction pattern serving as the mask, a resist pattern 12 is formed in Step (10) of FIG. 26, in the same manner as with Step (7). Next, removing the resist pattern 12 following dry etching yields a diffraction optical device with an eight-stepped stair-like pattern 13, as shown in Step (11) of FIG. 26.
Thus, diffraction optical devices or molds having a stair-like cross-sectional diagram, referred to as “binary optics”, can be manufactured by exposure, lithography process based on etching technique, and film-forming technique, these being used in semiconductor manufacturing art. The optical capabilities of such diffraction optical devices are exhibited based on the recessed and projected stair-like form which is formed on the substrate, so the diffraction efficiency thereof is affected by the form, i.e., the depth, width, and cross-sectional form of the formed steps.
In the case of sequentially using such double-fold masks to form a diffraction optical device with a plurality of steps, an ideal 8-step formation A can be manufactured using three masks 17a through 17c, as shown in FIG. 27, so long as there are nonalignment errors or dimensional errors.
However, with the above example, in manufacturing technique using a plurality of masks, margin of error in the form of the steps owing to margin of error in alignment markedly deteriorates diffraction efficiency, and once such an error in form is created it cannot be restored, which consequently raises costs. In reality, it is impossible to completely do away with all such alignment margin of error and dimensional margin of error, so in the event that there is offset in the alignment of the masks 17a through 17c shown in FIG. 28 to the degree of r1 and r2, a diffraction optical device is formed in the form of B instead of the intended form A. Accordingly, the optical capabilities such as diffraction efficiency greatly deteriorate, and in addition, in the event that dimensional errors occur in each layer, the deterioration of optical capabilities decreases even further.
For example, in the event that quartz is used as the substrate and an ideal 8-stepped form as shown by form A is formed with a minimum line width of 0.35 μm, step height d of each of 61 nm, and usage wavelength of 248 nm, the logical diffraction efficiency obtained by subtracting loss from reflection is 95%. On the other hand, in the event that the margin of error r1 between reticle 17a and reticle 17b is 80 nm, for example, and the margin of error r2 between reticule 17a and reticule 17c is 30 nm, the diffraction efficiency drops by 15% to 80% even without taking reflection into consideration, and these results have been confirmed in actual measurement and simulation.
Also, in order to form a multi-stepped diffraction optical device with a similar method, resist processing following a plurality of times of exposing and developing is carried out, and a 16-stepped stair-like diffraction optical device can be manufactured using quartz as the substrate, with, e.g., a minimum line width of 0.35 μm, step height d of each of 30.5 nm, and usage wavelength of 248 nm. In the case of an ideal 16-stepped form, the logical diffraction efficiency obtained by subtracting loss from reflection is 99%, but in the event that margin of error of alignment is included in this, the diffraction efficiency drops far below that of the 8-stepped form.
Thus, control of the dimensions and alignment of resist pattern is in actual practice is quite difficult, reproducibility cannot be obtained, and consequently, the steps become narrower or wider than intended, so that grooves and protrusions non-existent in the ideal step formation are formed, and is problematic in that the optical capabilities of the diffraction optical device markedly deteriorate.
Also, while electron beam drawing does away with a margin of error in alignment, the immense amount of drawing creates a problem of inefficient manufacturing through-put.
Further, generally, in the case of using glass for the diffraction optical device, there is the need to form the resist thickly in order to obtain diffraction patterns with deep steps, since the etching speed is slow and the speed of etching the resist and the glass is approximately the same, and consequently, substances generated by reaction in etching at the deep portions of the grove cannot find a way out in the event that the resist is thick, having an ill effect on the cross-sectional form and disrupting the rectangular form of the side walls.